When a sinusoidal voltage is applied to a linear load, the current drawn by the load is a sinusoidal current at the same frequency as the voltage (although usually not in phase with the voltage). In contrast, when a sinusoidal voltage is applied to a non-linear load, such as a rectifier, the current drawn by the load is non-sinusoidal. Instead, the current waveform is complex and consists of a series of multiple sinusoidal signals that start at the fundamental frequency of the power system and occur at integer multiples of the fundamental frequency known as harmonics. Harmonic frequencies in a power system are a frequent cause of power quality problems. In addition, power systems designed to function at the fundamental frequency of the system may experience unsatisfactory operation and/or failure when subjected to voltages and currents that contain substantial harmonic elements.
A variable frequency drive is a solid state electronic power converting device used for controlling the rotational speed of an alternating current (AC) electrical motor by controlling the frequency of the electrical power supplied to the motor (as is known, the synchronous speed of an AC motor is determined by the frequency of the AC supply and the number of poles in the stator winding). Typically, a variable frequency drive first converts an AC input power to a DC intermediate power using a rectifier circuit. The DC intermediate power is then converted to a quasi-sinusoidal AC power using an inverter switching circuit.
As noted above, variable frequency drives usually include rectifiers in their front ends. As also noted above, rectifiers, being non-linear, produce harmonics and, sometimes, reactive power. It is always desirable to have filters between the voltage sources and the rectifiers so that the sources are protected from damage by the harmonics and/or reactive power. Traditionally, passive filters have been used for this purpose where they have been tuned to certain harmonic frequencies so that they behave as harmonic sinks. At the same time, the passive filters provide reactive power naturally, although the amount of the reactive power they provide is typically not enough to provide effective compensation for the reactive power described above. While a passive filter performs well in filtering a particular order of harmonics, it tends to couple with the power line impedance to form an oscillation circuit and thus introduces oscillating current. Recently, active filters have also been used for this purpose. Active filters use power electronic devices, such as electronic switches like Insulated Gate Bipolar Transistors (IGBTs), and switch them on and off intelligently so that they compensate for the harmonic current(s) and reactive power.
U.S. Pat. No. 5,321,598 describes an active filter that is used to filter harmonics. It injects current that is 180 degrees out of phase from the load harmonic current to compensate for that harmonic current. U.S. Pat. No. 5,355,295 describes a voltage injection plus current injection topology to compensate for both current harmonics and voltage harmonics. U.S. Pat. No. 5,548,165 describes a hybrid filter that has voltage sharing topology, which is suitable when the voltage rating of the power electronic semiconductor switches is lower than the system voltage. U.S. Pat. No. 4,812,669 and JP 63033006 use hybrid filters to filter harmonics generated by a non-linear load. However, these approaches use passive filters to filter high-order harmonics and active filters to filter low-order harmonics. Such a configuration has merits in that the requirement for the switching frequency of the power electronic semiconductor switches is lower for filtering low-order harmonics.
While the approaches described above have been effective in some applications, there is room for improvement in the area of harmonic filtering, reactive power compensation and/or oscillation dampening for power systems, particularly in high power applications where the electric current requirement for the power electronic devices used therein is one of the driving factors.